Natural rubber latex (NRL) is the material of choice in many applications requiring soft, thin-walled elastic articles, such as medical examination and scientific gloves. NRL articles typically are made by dipping a mandrel, pre-coated with a coagulant, into an aqueous rubber emulsion. To produce NRL gloves which are adequately strong, but which do not have pinholes, the dipped NRL gloves must be about 0.18 to about 0.20 mm thick. Although gloves made from NRL possess an excellent combination of flexibility, high elongation at break, tensile strength and a low degree of creep, in recent years the use of NRL has been under attack because of increased instances of allergic reactions caused by proteins and vulcanization accelerator residues present in NRL.
The manufacture of medical exam and scientific gloves from polyurethane is known to those skilled in the art. For example, U.S. Pat. No. 4,463,156, issued to McGary, Jr., et al., discloses a soft, low modulus, non-crystalline segmented polyurethane with a 100% modulus less than about 250 psi, initial tensile set less than about 30% and tensile strength of about 3500 to 6500 psi. McGary, Jr. et al. teach balancing the percent hard segment and the degree of cross-linking within the ranges of 14 to 25% hard segment and 5,000 to 30,000 molecular weight per cross-link in a segmented polyurethane. The polyurethane of the 156 patent is said to produce a flexible glove for use by surgeons and others which is easily donned and comfortable on the hand.
A waterborne dispersion of a polyurethane polymer said to be useful as a film having improved mechanical properties is disclosed by Snow, et al. in U.S. Pat. No. 6,017,997. The film is said to have properties comparable to rubber including, a percentage elongation greater than 700%, a tensile strength greater than 3,500 psi, a 100% modulus below 450 psi, a 300% modulus below 700 psi, and a 500% modulus below 1,500 psi. This film can be prepared in the absence of a solvent, thus making it attractive for a variety of protection products for medical and industrial applications such as gloves, condoms, catheters, and the like. However, the films of Snow et al. are crosslinked by the urethane reaction; therefore, the residual NCO may cause allergic reactions on human skin.
Papalos et al., in U.S. Statutory Invention Registration No. H 1,712, disclose radiation-curable compositions containing monoalkyl or mono-aralkyl ether di-acrylates or di-methacrylates of particular ethoxylated and/or propoxylated polyols. These polyol derivatives substituted with hydrophobic ether functionalities are said to constitute UV-curable diluents that can be isolated in excellent yields with minimal losses during the manufacturing process. The UV-curable diluents of Papalos et al. are said to be useful in radiation-curable compositions.
U.S. Pat. No. 4,876,384 issued to Higbie et al., teaches reactive diluents for radiation curable compositions. The diluents are lower alkyl ether acrylates and methacrylates of particular alkoxylated and non-alkoxylated polyols. Examples are mono-methoxy trimethylolpropane diacrylate, mono-methoxy neopentyl glycol monoacrylate and mono-methoxy, ethoxylated neopentyl glycol monoacrylate having an average of about two moles of ethylene oxide.
No mention is made in either of these disclosures of making a coating, adhesive, sealant or elastomer from the reactive diluent by combining it with an isocyanate to form a polyurethane.
Polyurethane is typically made by reacting a polyol with an isocyanate. The majority of polyoxyalkylene polyether polyols are polymerized through base catalysis. For example, polyoxypropylene dials are prepared by the base-catalyzed oxypropylation of a difunctional initiator such as propylene glycol. During base-catalyzed oxypropylation, a competing rearrangement of propylene oxide to allyl alcohol continually introduces an unsaturated, monofunctional, oxyalkylatable species into the reactor. The oxyalkylation of this monofunctional species yields allyl-terminated polyoxypropylene monols. The rearrangement is discussed in BLOCK AND GRAFT POLYMERIZATION, Vol. 2, Ceresa, Ed., John Wiley & Sons, pp. 17-21. Unsaturation is measured in accordance with ASTM D-2849-69 “Testing Urethane Foam Polyol Raw Materials,” and expressed as milliequivalents of unsaturation per gram of polyol (meq/g). Due to this continual creation of allyl alcohol and its subsequent oxypropylation, the average functionality of the polyol mixture decreases and the molecular weight distribution broadens. Base-catalyzed polyoxyalkylene polyols contain considerable quantities of lower molecular weight, monofunctional species. In polyoxypropylene dials of 4,000 Dalton (Da) molecular weight, the content of monofunctional species may be between 30 and 40 mole percent. In such cases, the average functionality is lowered to 1.6 to 1.7 from the nominal, or theoretical functionality of 2.0. In addition, base-catalyzed polyols have a high polydispersity (Mw/Mn) due to the presence of the substantial, low molecular weight fractions.
Lowering unsaturation and the attendant high monol fraction in polyoxypropylene polyols has been touted as a means of providing polyurethane compositions with improved properties. Often, formulations must be chosen to balance conflicting properties. For example, increases in tensile strength are often accompanied by a decrease in elongation.
Therefore, a need exists in the art for a polyol that will provide prepolymers that can be used to make thin films having the desired properties of natural rubber latex (NRL) including low modulus, high elongation, high tensile strength and solvent resistance, without the drawbacks associated with NRL such as allergic reactions. Further, it would be desirable if the polyol could provide UV-curable polyurethane compositions having a low modulus, high elongation, high tensile strength and solvent resistance that are suitable for use in or as coatings, adhesives, sealants, elastomers and the like.